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Default to your own packaging, and reach for carrier packaging only where you have checked that the particular parcel prices better in it. The reason is that the Package drop-down is not a note about what the box is — it is an input to the rate, and choosing a carrier’s flat-rate packaging turns off pricing paths your own box would have qualified for.
Two things at once, and conflating them is where this question goes wrong.
First, it sets the dimensions on the order. That is its primary job. The drop-down holds ShipStation’s own preset packages — mostly carrier-specific ones like Flat Rate boxes and One Rate boxes — followed by whatever custom packages you have added. Picking a named package writes that package’s saved dimensions onto the order. Picking the generic Package entry writes nothing and leaves you to type the dimensions in yourself.
So the difference between generic Package and one of your own custom packages is not what the carrier sees. It is who supplies the numbers: you, per order, or a definition you saved once.
Second, on the services that require it, it tells the carrier the box is theirs. This is the narrower job, and it only applies to a minority of services.
Some carrier services are priced on the assumption that you are shipping in the carrier’s own packaging, and the discount that service exists to offer is conditional on that. USPS Priority Mail Flat Rate is the clean example: the price is a fixed number for a USPS-supplied box, and you get it by selecting the matching Flat Rate package type in ShipStation. The pairing is tight — Flat Rate packaging only appears once you select USPS Priority as the service, using it with any other USPS service returns an error, and the Flat Rate package types have to be enabled in your Packages settings before they appear at all.
The thing to hold on to is that this is a declaration, not a measurement. The carrier has to be told the box is one of theirs, and the only way it hears that is if you tell ShipStation first. Typing the Flat Rate box’s exact dimensions by hand does not do it — dimensions are three numbers, and nothing in them says whose box they came from. ShipStation relays what you selected, so if you selected nothing, there is nothing to relay.
Which makes the failure quiet. Ship a Priority parcel in a genuine USPS Flat Rate box, enter its dimensions correctly, and skip the Flat Rate package type, and you pay a measured rate for a box that had a fixed price available. Every number on the order is correct and the price is still wrong, so nothing downstream flags it.
Every other service is not asking about your packaging. It is asking for accurate dimensions and weight, because those are what it prices from. That is the majority of what you ship, and it is where custom packages earn their place — not as a different rating path, but as a reliable way of getting the right dimensions onto the order without a person typing them each time.
Weight sits outside all of this and still has to be right. A package selection supplies dimensions; it never supplies weight. That is as true on a flat-rate parcel as on a measured one — flat-rate services have their own eligibility limits, and the weight on the order is what the carrier bills and re-rates against either way.
One clarification, because “custom” sounds bespoke and is not. A custom package is not a one-off you define per order. It is one entry in the standing list of boxes your fulfilment centre actually uses — the 12 × 9 × 4 mailer, the small double-wall carton — added once and then selected by name.
Getting that list into ShipStation makes your dimensions accurate. It does not, on its own, make them cheap, and the two get conflated.
Carriers bill the greater of a parcel’s actual weight and its dimensional weight, so on anything light for its size the box sets the price rather than the contents — worth understanding on its own terms. What that implies for your list is that the saving is not in having packages defined. It is in having the right sizes: a set that, across your real order mix, gives you the smallest box each shipment genuinely fits in, so fewer parcels cross into dimensional weight and the ones that do cross by less.
An operation running three box sizes ships most orders in a carton bigger than they need, and every dimension on those orders is perfectly accurate. Accuracy and cost are separate problems, and a package list only solves the first one by itself.
ShipStation sells shipping supplies — boxes, mailers, labels — through a separate store. It is worth saying plainly that buying them changes nothing about how a parcel is rated, because the naming invites the opposite conclusion: they sell the box, the drop-down is called Packages, so surely selecting one does something.
It does not. A box bought from ShipStation is your packaging, in exactly the way a box bought from any supplier is. You add it to your package list, it writes its dimensions onto the order, and the carrier prices it from those dimensions like any other parcel. There is no rate attached to where the cardboard came from.
Carrier packaging is the narrow exception, and it is narrow because the carrier is giving you a fixed price in exchange for using their box on their service. Nobody else is offering that, so nobody else’s box has anything to declare.
Which puts the buying decision back where it belongs: whether a box is a good price for the size, and whether that size is one your order mix actually needs.
FedEx defines the program. ShipStation is where you declare you are shipping under it.
FedEx sets the packaging terms: use one of the packages they list, or your own that meets the same requirements. Miss any of them — bulge past the cubic volume, go over the weight — and FedEx reclassifies the shipment and the flat rate is gone. The requirements are published on their One Rate page.
ShipStation says the same thing from its side. The FedEx packaging type is required to calculate the One Rate price correctly, and you can still use your own box or packaging if it meets the dimension restrictions.
Which is the relay in one line. The specification, the rate and the reclassification are all FedEx’s. Selecting the package type is how you tell ShipStation to tell them.
Rate Shopper does not close this gap, and the reason is structural rather than an oversight.
It requests rates from the carriers for the services in its rule, then evaluates the returned rates and delivery times against that rule. It compares what came back. It does not recompute anything, and it varies the service rather than the package — ShipStation states plainly that Rate Shopper only compares services and does not compare different package types.
So the package on the order is an input to every rate request Rate Shopper makes, and the one variable it never changes. Point it at a parcel already declared as a Flat Rate box and it will find the cheapest service for a Flat Rate box, correctly and confidently, without ever pricing that parcel in your own carton. There is no error and no flag, because nothing went wrong.
That is the same boundary flat-rate programs run into generally: packing and rating are one decision, and the platform takes them in sequence.
Choosing the cheaper of two boxes is a smaller question than choosing the right box in the first place. For single-item orders with stable packaging, the above is the whole job. For multi-item orders the box is not knowable until you work out what fits, which is a different problem and not one a package drop-down answers.
Knowing how a carrier prices a package is useful. The harder problem is making the right packing and rate decision on every order, when the correct answer changes with the contents, the destination and the rate card.
String builds and maintains that logic inside existing ShipStation accounts, for operations shipping roughly 6,000 orders a month or more. Below that, ShipStation's native automation and accurate product dimensions will get you most of the way there.
Eight patterns that show up when an operation needs custom shipping logic
